My oldest cat that has allergies eats canned Nature's Variety Instinct in Rabbit (it's grain free). She get's 1/3 can (5.5oz can) in morning and 1/3 can at night. In the afternoon when I get home from work she get's one Nature's Variety Raw RAbbit medallion mixed with just a little canned because she's not crazy about the raw.
My younger cat eats mostly Merrick Canned food. I like to buy the variety pack so she eats something different every day. She gets 1/2 can (3.2oz can) in morning and 1/2 can in evening (sometimes mixed with a little dry - Eagle Pack Holistic currently). In the afternoon she gets one Nature's Variety Raw medallionin rabbit or venison. I also gave her a small piece of a raw chicken neck last night and she ate some of it.
You might try mixing some grain free canned and raw together to do something different. cats are carnivors and raw meat is really good for them (as long as they are healthy and not immunocompromised). If you don't like the idea of raw, you can supplement cooked meats with raw carrots, potatos, peas, fruits (no grapes or raisins). Cats can be really picky eaters though so a lot of cats don't like to try really different things that they aren't used to. But it's worth a try. That's why i like to feed my younger cat a variety of the merrick canned foods - so she's not eating the same thing every day. I'm also trying to get some green tripe sometimem soon so I can give her that sometimes as it's very nutritious.
Be careful. Any change is actually not doing them much of a favor. It just upsets their belly. They don't really need the variety that humans do. Once you find something they like, they are happier and healthier to just stick with it. Even a can of kitten food for a treat caused diarrhea in my Allie. I felt bad because she had an Upper Respiratory Infection and I have to keep squirting antibiotic liquid into her mouth so I thought she deserved a treat. I knew better, i shouldn't have given it to her but she loved it SO much. Big Mistake!! Ugh, stinky potty!! But on the flip side of that, my friend had a cat that shared whatever he was eating. She especially liked American Cheese slices and would come running when she heard the package crinkle. She was fat but she lived to the ripe old age of 22.